GABA is one of the amino acids widely present in the nature and contained in sprouted brown rice, tea, vegetables and grains. GABA is an inhibitory neurotransmitter present in the brain and the spine of mammalians including humans and known to have physiological effects such as antihypertensive effect, mood-stabilizing effect, anti-stress effect, alcohol metabolism acceleration effect, brain metabolism acceleration effect and anti-obesity effect.
Accordingly, in expectation of improving hypertension and obtaining a tranquilizing effect, it has been proposed to take food enriched in GABA. To increase the content of GABA in food, the following methods have been developed: for example, a method of increasing the content of GABA by processing tea leaves in anaerobic conditions such as commercially available Gabaron tea and a method of markedly increasing the content of GABA and free amino acids by soaking, for example, rice germ in water (Non-Patent Document 1). However, these methods have problems in that GABA is diluted when extraction of tea leaves is performed with hot water and that a large amount of germ must be collected. These problems still remain unsolved.
On the other hand, foods enriched in GABA have been developed by microbiological fermentation methods as shown in the following reports:
a method of obtaining GABA in a high concentration by inoculating two types of lactobacilli (Patent Document 1);
a method of obtaining GABA by lactic fermentation of glutamic acid (a starting material for GABA), which is produced by processing a milk product with a protease (Patent Document 2);
a method using a lactobacillus isolated from Korean pickle, kimchi (Patent Document 3);
a method using a lactobacillus isolated from crucian sushi (Patent Document 4);
a method of attaining a low sodium content by use of glutamic acid and two or more types of lactobacilli (Patent Document 5); and
a method of producing soybeans added with GABA by use of a lactobacillus having glutamic decarboxylase (Patent Document 6).    [Patent Document 1] JP-A-2000-308457    [Patent Document 2] JP-A-2001-120179    [Patent Document 3] JP-A-2003-70462    [Patent Document 4] JP-A-2005-102559    [Patent Document 5] JP-A-2005-198578    [Patent Document 6] JP-A-2004-187501    [Non-Patent Document 1] Chemistry and Biology Vol 33, No. 4, 1995